Fluorine atoms (radicals) reacts spontaneously with silicon at room temperature so that it is not possible to produce an anisotropic etching profile in a silicon substrate unless the substrate is cooled to −140° C. when forming a deep hole in the substrate in a vertical direction with a lateral wall standing perpendicularly relative to the substrate, suppressing the hole from extending transversally. Therefore, when etching silicon, using fluorine gas, it is necessary to suppress isotropic etching by forming an anti-etching or protecting film on the lateral wall for the purpose of producing a desired anisotropic etching profile in the silicon.
Anisotropic etching methods of forming an anti-etching film on the lateral wall to produce an anisotropic etching profile in silicon are known (see Patent Documents 1 and 2).
With the anisotropic etching method described in Patent Document 1, polymerization steps and etching steps are conducted alternately and continuously to form a polymer layer in each polymerization step on the surface exposed by the immediately preceding etching step to protect the lateral surface against etching in the succeeding etching step.
With the anisotropic etching method described in Patent Document 2, there are alternately and repeatedly conducted a step of conducting anisotropic etching by introducing mixture gas containing etching gas (SF6) and passivation gas (CHF3, C4Fn, etc.) into a process chamber, exciting it by means of electromagnetic radiation and, at the same time, applying a high bias voltage to the substrate, and a step of producing unsaturated monomers in plasma by exciting the mixture gas by means of electromagnetic radiation and, at the same time, forming a protective polymer coat on the lateral wall having an exposed surface to be subjected to etching by applying a low bias voltage to the substrate.                Patent Document 1: U.S. Pat. No. 5,501,893        Patent Document 2: Jpn. Pat. Appln. Laid-Open Publication No. 2000-323454        
However, with the prior art method described in Patent Document 1, the temporal ratio of each etching step and the immediately succeeding polymerization step depends on the velocity of the gas mixture being introduced and changes from time to time to adversely affect the uniformity of etching because different gas mixtures are repeatedly used in the etching steps and the polymerization steps. Additionally, a complex apparatus is required to realize the method because solenoid valves have to be used to switch from one of the two gas mixtures to the other and vice versa. Still additionally, a problem of generation of particles arises due to the gas mixture introduced in the polymerization steps.
With the prior art method described in Patent Document 2, it is necessary to shift the bias voltage being applied to the substrate from the level good for the etching step to the level good for the polymerization step and vice versa and hence requires a control system having a complex configuration to consequently raise the cost of the apparatus for realizing the method. Additionally, a problem of generation of particles arises due to the gas mixture introduced in the polymerization steps. Still additionally, a high energy RF power has to be applied for forming a polymer film to raise the energy cost.
Furthermore, with the method of forming an anti-etching film, using passivation gas, only several % to a little more than 10% of the introduced passivation gas takes part in the film forming activity. The passivation gas that does not take part in the film forming activity is exhausted from the vacuum chamber. Since passivation gas shows a high atmosphere warming coefficient and constitutes a negative factor for environment protection, the exhausted passivation gas needs to be collected and processed. The operation of processing passivation gas involves large cost and hence it is not desirable to use passivation gas.
A high bias etching method that uses HBr is also known for obtaining an anisotropic etching profile. However, it is not possible to etch silicon deep with this method because of the problems it entails including that a high etch selectivity cannot be expected for a mask and it is not suited for forming micro electromechanical systems and manufacturing micro electronic apparatus, although it can suitably be used for more microscopic patterns.